Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen, or steam, or a combination of both to form synthesis gas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch (FT) synthesis, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Those molecules containing oxygen in addition to carbon and hydrogen are known as oxygenates. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons and are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). The molecules react to form hydrocarbons while confined on the surface of the catalyst. The hydrocarbon products then are desorbed from the catalyst and can be collected. H. Schulz (Applied Catalysis A: General 1999, 186, p. 3) gives an overview of trends in Fischer-Tropsch catalysis.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors, either placed in series, in parallel or both. Common reactors include packed bed (also termed fixed bed) reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors comprising catalytic particles sometimes called “slurry reactors”, “slurry bed reactors” or “slurry bubble column reactors,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Some of the principal advantages of gas-agitated multiphase reactors or slurry bubble column reactors (SBCRS) for the exothermic Fischer-Tropsch synthesis are the very high heat transfer rates, and the ability to remove and add catalyst online. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer Tropsch reactors.
It is clear from the prior art that the performance of a SBCR is a combined result of reaction kinetics, heat and mass transfer, and multiphase hydrodynamics. Jackson, Torczynski, Shollenberger, O'Hern, and Adkins (Proc. Annual Int. Pittsburgh Coal Conf. 1996, 13th (Vol 2), p. 1226) showed experimental evidence of the increase of gas hold up with increase in the inlet superficial velocity in a SBCR for Fischer Tropsch synthesis. Krishna, DeSwart, Ellenberger, Martina, and Maretto (AIChE J. 1997, 43(2), p. 311) measured experimentally the increase in gas holdup with an increase in the gas velocity and solids concentration in a slurry bubble column in churn turbulent regime. Letzel, Schouten, Krishna and van den Bleek (Chem. Eng. Sci 1999, 54, p. 2237) developed a simple model for gas holdup and mass transfer at high pressure in a slurry bubble column. Numerically, Sanyel, Vasquez, Roy, and Dudukovic (Chem. Eng. Sci. 1999, 54, p. 5071) and Pan, Dudukovic, and Chang (Chem. Eng. Sci. 1999, 54, p. 2481) showed examples of computational fluid dynamic modeling and optimization of a slurry bubble column reactor irrespective of the chemistry. Wu and Gidaspow, (Chem. Eng. Sci 2000, 55, p. 573) show examples of computational fluid dynamics simulations of hydrodynamics of Slurry Bubble Column processes.
Much previous work has been aimed at optimization of the slurry bubble column system for Fischer Tropsch and other chemistries. Stem et al. (Ind. Eng. Chem. Process Des. Dev.1985, 25, p. 1214) developed an axial dispersion model for describing the performance of gas agitated multiphase reactor used for Fischer-Tropsch synthesis. Saxena (Cat. Rev. -Sci. Eng. 1995, 37, p. 227) gives a review of the detailed experimental findings and theoretical models for the design of a Fischer Tropsch SBCR.
Considerable patent literature examines optimization of Fischer Tropsch Slurry Bubble Column reactors (SBCRs). U.S. Pat. No. 5,252,613 presents a method for improving catalyst particle distribution by introducing a secondary suspending fluid. U.S. Pat. No. 5,348,982 discloses one mode of operation for an SBCR. U.S. Pat. No. 5,382,748 shows the use of a vertical downcomer to promote the uniform catalyst distribution. U.S. Pat. No. 5,961,933 and U.S. Pat. No. 6,060,524 disclose that optimal operation can be obtained by introduction of liquid recirculation. Despite the significant level of research, there remains a need for an optimized Fischer Tropsch reactor and reactor configuration.
It is noted by Deckwer (Chem. Eng. Sci. 1976, 31, p. 39) that the gas dispersion is important in bubble columns of diameters greater than 0.5 m, as it may have a strong influence on conversion. It is found that the gas dispersion is a function of the gas holdup, superficial gas velocity, and reactor diameter. In the gas-liquid-solid three-phase reactor, the gas holdup depends on many factors such as gas and liquid velocities, gas distributor design, column geometry, physical properties of the gas and liquid, particle concentration, and reactor internals. Therefore, the gas dispersion coefficient is also a complicated function of these design and operating parameters. Usually, it is necessary to perform an in situ measurement to determine the dispersion coefficient at a given condition.
It is known that the flow patterns of individual phases can affect the reactor performance. Plug flow and well-mixed flow are two extreme flow patterns for reactor systems. In plug flow, there is no backmixing within the reactor, and the composition of the reactants varies with the position within the reactor. By contrast, in a well-mixed system, the composition of the slurry is similar at every point within the reactor. The dimensionless Peclet number, Pe, can be used to represent the degree of backmixing in plug flow. In general, it can be said that higher Peclet numbers indicate less backmixing, i.e. approaching plug flow, while better-mixed flow regimes are associated with lower Peclet numbers. Hence, the highest Peclet numbers will occur when flow in the reactor approximates plug flow.
A Peclet number can be calculated for each phase in a slurry bubble column reactor. Thus, the type of flow of the gas phase in the reactor can be described by the gas Peclet number, which has the form PeG=UGL/DG, where UG is the superficial gas velocity, L is the expanded slurry bed height, and DG is the gas dispersion coefficient. Superficial gas velocity is defined herein as the total inlet gas volumetric flow rate at reactor inlet temperature and pressure divided by the cross sectional area of the reactor vessel excluding the area occupied by any internals and is sometimes referred to as “inlet superficial gas velocity.” The gas dispersion coefficient, DG, is a function of the superficial gas velocity, gas holdup, and the reactor diameter. For large scale industrial bubble columns, the axial dispersion coefficients of gas and liquid phases can be calculated using correlations proposed by Field and Davidson (Trans. I. Chem. E 1980, 58, p. 228). The change of the gas Peclet number with the superficial gas velocity at three reactor aspect ratios is shown in FIG. 1. As shown in FIG. 1, the gas Peclet number decreases with the increase of the superficial gas velocity for a given reactor aspect ratio, defined by the ratio of reactor height over reactor diameter (L/D). Field and Davidson's article also presents the correlation for the liquid Peclet number in bubble columns with liquid circulation. The liquid Peclet numbers for the conjoined reactor system of this invention are calculated using these correlations, and the results are presented in FIG. 2. FIG. 2 shows the liquid Peclet number changes with the superficial gas velocity at two liquid circulation velocities. The liquid circulation velocity is defined as the liquid linear velocity in a reaction zone. In the figure, solid lines show the results with positive liquid velocity which corresponds to the upward liquid flow while dash lines show the results with negative liquid velocity which corresponds to the downward liquid flow.
It has been found that Fischer-Tropsch fluidized bed reactors operating at conditions approaching plug flow regime typically provide higher productivity for a given gas superficial velocity than reactors operating with a higher degree of backmixing. However the FT slurry bed reactors operating at conditions approaching plug flow regime typically are at low superficial gas velocities and tend to suffer from uneven distribution of the gas, liquid and solid phases and difficulty of temperature control. In particular, one characteristic of many conventional slurry Fischer-Tropsch reactors is that the flow through the reactor tends to have core-annular characteristics in that an outer, annular region of the reactor will have a much lower gas content than that in the inner region. This core-annular flow reduces the effective volume of the reactor because the entire reactor is not operating at the most efficient reaction conditions. The existence of a core-annular flow also causes an accumulation of water in annular regions of the reactor. Further details relating to Peclet number can be found in co-pending and commonly owned U.S. patent application Ser. No. 10/023,258 filed Dec. 14, 2001, and entitled “Slurry Bed Reactor Operated in Well-Mixed Gas Flow Regime,” which is incorporated herein by reference.
Hence, despite significant research in the field of fluidized bed reactors, a need persists for a reactor system that will provide high productivity while also providing more even distribution of its gas, solid and liquid phases and allowing a high degree of temperature and reaction control.